In wavelength division multiplexing (hereinafter, referred to as “WDM”) optical communication, as a multiplexed wave including a signal wave of a wavelength of 1.5 μm and a signal wave of a wavelength of 1.3 μm is transmitted through one optical fiber, large-capacity and high-speed optical communication is realized. Meanwhile, in WDM optical communication, a device for demultiplexing the multiplexed wave so that an individual signal wave is output from a designated port is necessary. For example, Non-Patent Documents 1 to 4 described below propose as a demultiplexing device, a planar waveguide device utilizing optical bloch oscillations (hereinafter, referred to as “OBO planar waveguide device”).
Non-Patent Document 1: R. Morandotti, U. Peschel, and J. S. Aitchison, “Experimental Observation of Linear Nonlinear Optical Bloch Oscillations,” Physical Review Letters, Vol. 83, No. 23, pp. 4756-4759 (Dec. 6, 1999).
Non-Patent Document 2: T. Pertsch, P. Dannberg, W. Elflein, and A Brauer, “Optical Bloch Oscillations in Temperature Tuned Waveguide-Arrays,” Physical Review Letters, Vol. 83, No. 23, pp. 4752-4755 (Dec. 6, 1999).
Non-Patent Document 3: U. Peschel, T. Pertsch, and F. Lederer, “Optical Bloch oscillations in waveguide arrays,” Optics Letters, Vol. 23, No. 21, pp. 1701-1703 (Nov. 1, 1998).
Non-Patent Document 4: A. Sakaki, G. Hara and T. Baba, “Propagation Characteristics of Ultrahigh-Δ Optical Waveguide on Silicon-on-Insulator Substrate,” Japanese Journal Applied Physics., Vol. 40, No. 4B, pp. L383-L385 (Apr. 15, 2001).
FIG. 48 shows an OBO planar waveguide device disclosed in Non-Patent Document 2. As shown in FIG. 48, a conventional OBO planar waveguide device 300 includes a substrate 301 composed of SiO2 (silicon dioxide) and glass, a macromolecular clad layer 302 preventing a propagating signal wave from leaking from an optical waveguide, a plurality of optical waveguides 303 for propagation of light formed from a macromolecular material, an input port 304 for a signal wave and an output port 305 for a signal wave in the plurality of optical waveguides 303, and a heater 306 and a heat sink 307 for controlling gradient of temperature distribution of substrate 301.
In addition, FIG. 48 also shows a path 308 of the signal wave that propagates through the plurality of optical waveguides 303 while OBO is being caused. As shown in FIG. 48, a direction in which the plurality of optical waveguides 303 are aligned is assumed as an X-axis direction, a direction in which each optical waveguide 303 extends is assumed as a Y-axis direction, and a direction perpendicular to the X-axis direction and the Y-axis direction is assumed as a Z-axis direction. It is noted that a thickness and a size of substrate 301 are not specified in Non-Patent Document 2.
A method of manufacturing OBO planar waveguide device 300 will now be described. Initially, a macromolecular material for forming optical waveguide 303 is deposited on substrate 301. A resist film is applied onto the macromolecular material. Thereafter, a pattern of optical waveguide 303 is formed on the aforementioned resist film using electron beam lithography or photolithography. Using the resist film having the pattern formed as a mask, the aforementioned macromolecular material is etched so as to obtain optical waveguides 303 of OBO planar waveguide device 300. Thereafter, macromolecular clad layer 302 is deposited to cover substrate 301 and optical waveguides 303. Then, heater 306 is attached to one side end portion in the X-axis direction of substrate 301, and heat sink 307 is attached to the other side end portion in the X-axis direction of substrate 301.
According to OBO planar waveguide device 300 described above, temperature difference per unit length in the X-axis direction of substrate 301 is controlled by regulating the temperature of heater 306. In addition, difference in a refractive index per unit length in the X-axis direction of optical waveguide 303 is varied depending on the temperature difference. OBO originates from the difference in the refractive index. Generally, in OBO planar waveguide device 300 having optical waveguide 303 formed from the macromolecular material, optical waveguide 303 at a position where substrate 301 attains to a high temperature has a refractive index lower than that of optical waveguide 303 at a position where substrate 301 attains to a low temperature.
While aforementioned OBO planar waveguide device 300 is used, gradient of temperature distribution in the X-axis direction of substrate 301 is generated due to a function of heater 306 and heat sink 307. Then, a plurality of signal waves different in wavelength are input to OBO planar waveguide device 300 such that a peak of signal wave intensity is located at one prescribed input port 304. Accordingly, each of the plurality of signal waves leaks from optical waveguide 303 through which it passes, and is coupled to adjacent optical waveguide 303. Consequently, OBO of the plurality of signal waves occurs in the X-axis direction while the signal waves propagate in the Y-axis direction. Generally, as the wavelength of the signal wave is higher, the amplitude of OBO is greater. In addition, amplitude of OBO tends to be smaller as the difference in the refractive index per unit length in the X-axis direction of optical waveguide 303, that is, gradient of distribution of the refractive index in the X-axis direction of optical waveguide 303, is greater. In other words, the amplitude of OBO is different for each wavelength of the signal wave, depending on a characteristic of OBO.
Meanwhile, the multiplexed wave is obtained by superimposing a plurality of signal waves different in wavelength onto each other. Accordingly, path 308 of propagation through OBO planar waveguide device 300, of the multiplexed wave input to OBO planar waveguide device 300 is different for each wavelength. That is, the multiplexed wave is demultiplexed in OBO planar waveguide device 300. In addition, each demultiplexed signal wave is output from specific output port 305 as a single wave.
By regulating the temperature difference per unit length in the X-axis direction of substrate 301 using heater 306 and heat sink 307, the difference in the refractive index per unit length in the X-axis direction of optical waveguide 303 is controlled. By controlling the difference in the refractive index, magnitude of the amplitude of OBO of the demultiplexed signal wave can be regulated, to freely designate output port 305 for individual signal wave. Therefore, OBO planar waveguide device 300 can be utilized as a variable demultiplexer.
If OBO planar waveguide device 300 is employed as a demultiplexing device in WDM optical communication and if the signal wave in a band of 1.55 μm wavelength and the signal wave in a band of 1.3 μm wavelength propagate through optical waveguide 303 formed from the macromolecular material, however, propagation loss of the signal wave is significant. Therefore, OBO planar waveguide device 300 having optical waveguide 303 formed from a macromolecular material is not suitable as the demultiplexing device in WDM.
Meanwhile, in Non-Patent Document 4, an SOI (Silicon On Insulator) substrate is used as a method of reducing propagation loss of the signal wave in a band of 1.55 μm wavelength, and silicon is used as a material for the optical waveguide. In the SOI substrate, a silicon oxide (SiO2) layer and an Si layer are stacked successively on a silicon (Si) substrate. The Si layer having a thickness of approximately 0.3 μm and serving as an uppermost layer is etched to form the optical waveguide composed of Si (hereinafter, referred to as the “Si optical waveguide”). According to this OBO planar waveguide device, even when the signal wave in a band of 1.55 μm wavelength propagates through the Si optical waveguide, propagation loss of the signal wave is considerably small.
If the OBO planar waveguide device having the Si optical waveguide formed with the use of the SOI substrate is employed in order to reduce propagation loss in the OBO planar waveguide device as described in Non-Patent Document 4 above, power consumption of the heater for controlling the temperature difference (gradient of temperature distribution) per unit length in the X-axis direction of the Si substrate becomes greater.
Namely, in the aforementioned OBO planar waveguide device, thermal resistance of the Si substrate is lower than that of substrate 301 in conventional OBO planar waveguide device 300. Accordingly, if the heater is used consuming power as much as in the conventional example, the temperature difference per unit length in the X-axis direction of the Si substrate is made smaller. Therefore, in order to achieve the temperature difference per unit length in the X-axis direction of the Si substrate equal to that in the conventional example, power consumption greater than in conventional OBO planar waveguide device 300 is necessary.